Organic light-emitting diodes exhibit great potential in the application of display and illumination due to its low-cost, flexibility, high efficiency and high contrast. Thermally activated delayed fluorescence (TADF) possess the characteristics of metal-free and nearly 100% exciton utilization with low-cost, has been regarded as a promising candidate for next generation luminescent materials. Thereinto, the TADF sensitized fluorescence devices have been thoroughly studied which could incorporate the advantages of the high exciton utilization of TADF materials as well as the stability and perfect color quality of conventional fluorescence (CF) dopants. However, in the commonly applied co-phase sensitizing system, the severe Dexter energy transfer would induce the exciton loss in the devices, thus, developing a universal sensitizing system is critical for the evolution of sensitized OLEDs.
Recently, our group have proposed an efficient out-of-phase sensitizing system for the fabrication of white organic light-emitting diodes. Based on the novel blue TADF emitter with the emissive peak of 486 nm and the external quantum efficiency (EQE) of 26.6%, cooperated with green TADF emitter BDMAC-XT, red CF dopant DBP, succeeded the maximum EQE of 18.5% for out-of-phase sensitizing system. For contrast, the co-phase sensitizing system could only reach the EQE of 12.6%. We further introduced the assisted sensitizer TBRB and 4CzTPNBu to improve the Förster energy transfer for the better sensitization of DBP guest. Besides, through the insertion of TCTA interlayer between blue and green emissive layer, the balanced exciton distribution has been realized, thus generating the stable spectra at different luminance. Consequently, the devices with EQE over 23% and high color rendering index (CRI) of 87 have been fabricated. The robust electroluminescent performance and good color quality have provided the new instruction for the development of full fluorescence white OLEDs.
Ph. D. candidate Hao Liu in our group is the first author of this article, and the related paper is available online at Advanced Functional Materials (DOI: 10.1002/adfm.202103273).